CMPSCI 187: Programming With Data Structures

Practice for Second Midterm Exam

David Mix Barrington

Posted 12 October 2011

Directions:

• Answer the problems on the exam pages.
• There are seven problems for 100 total points. Likely scale is A=93, C=69.
• If you need extra space use the back of a page.
• No books, notes, calculators, or collaboration.

  Q1: 10 points
  Q2: 15 points
  Q3: 15 points
  Q4: 15 points
  Q5: 15 points
  Q6: 10 points
  Q7: 20 points
 Total: 100 points

Format is similar to first midterm, some text explaining the question types is omitted.

Here is a code base from previous assignments, that is assumed to be available throughout the exam:

public class Dog {
   private String name;
   private int age;
   // public get and set methods, two-parameter constructor
   }

public class SledDog extends Dog {
   private String breed = "Husky";
   // public get and set methods, three-parameter constructor
   }

public class LinearNode<T> {
   private T element;
   private LinearNode<T> next;
   // public get and set methods, zero-parameter and one-parameter constructors
   }

public class DogTeam {
   private LinearDog<SledDog> leadNode;
   private int size;
   // public get and set methods, zero-parameter constructor
   public void addToLead (SledDog newLead) {…}
   public SledDog removeLead ( ) {…}
   public void switchLastTwo ( ) {…}
   public SledDog removeYoungest ( ) {…}
   public int countHuskies ( ) {…}
   }

Code run before other code fragments:

   Dog ace = new Dog ("Ace", 6);
   Dog biscuit = new Dog ("Biscuit", 1);
   Dog cardie = new Dog ("Cardie", 3);
   Dog duncan = new Dog ("Duncan", 1);
   SledDog balto = new SledDog ("Balto", 92, "Husky");
   SledDog king = new SledDog ("King", 73, "Husky");
   SledDog buck = newSledDog ("Buck", 108, "Mixed");

public class Cell {
   private int x;
   private int y;
   private boolean isOpen;
   // public get and set methods, two- and three-parameter constructors
   }

public class SCell extends Cell {
   private boolean seen;
   // public get and set methods, constructors
   }

public class QCell extends SCell {
   private int distance;
   private QCell parent;
   // public get and set methods, constructors
   }

In Project #2 Maze is a two-dimensional array of SCells, in Project #4 of QCells. Both versions have methods path and isPath.

• Question 1: Java Concepts Briefly explain the difference between the two concepts in each pair (2 points each).
  • a) Queue (in java.util) and QueueADT (in L&C)
  • b) this (constructor) and this (object)
  • c) deque and dequeue
  • d) ArrayStack (in L&C) and ProjectThreeStack (in Project #3)
  • e) ArrayStack (in L&C) and LinkedStack (in L&C)

• Question 2: Software Engineering -- 5 points each
  • a) How would you modify the classes of Project 4 so that the user could specify a cell z and obtain a printout of the Maze, where in each cell's position there was a number indicating the distance from that cell to z?

  • b) Recall that L&C's maze program had a method called "traverse" that was supposed to return a boolean telling whether the maze had a path from the upper left corner to the lower right corner. How would you modify their method so that it is correct?

  • c) A dropout queue has a fixed capacity. If it is asked to enqueue an element when it is full, it drops the new element without throwing an exception. How would you modify either the circular array or the linked implementation of a queue (your choice) to make it a dropout queue?

• Question 3: Tracing Code -- indicate the output of each code fragment (5 points each)
  • a)

    
       // using L&C's code base
       QueueADT<Dog> q = new CircularArrayQueue<Dog>( ); 
       q.enqueue(biscuit);
       q.enqueue(king);
       Dog dog1 = q.first( );
       q.dequeue( );
       q.dequeue( );
       try{dog1 = q.peek( )}
       catch (EmptyCollectionException e) {dog1 = king;}
       System.out.println(dog1.getName( ));
    

  • b)

    
       // new method in DogTeam class
       public void merge (DogTeam other) {
          if (isEmpty( )) leadNode = other.leadNode;
          LinearNode ourLead = removeLead( );
          while (!other.isEmpty( )) 
             addtoLead(other.removeLead( ));
          addToLead (ourLead);}
       // then run this code
       DogTeam team1 = new DogTeam( );
       team1.addToLead (balto);
       team1.addToLead (king);
       team1.addToLead (buck);
       team1.switchLastTwo( );
       team2.add(king);
       team2.add(balto);
       team2.merge(team1);
       team1.removeLead( );
       System.out.println(team2.removeLead( ).getName( ););
    

  • c)

    
       // using the Maze class from Project #2
       String [ ] init = {"101", "010", "101"};
       Maze m = new Maze (3, 3, init);
       try {
          SCell sc = m.path(0, 0, 1, 1)[0];
          System.out.println(sc);}
       catch (ArrayOutOfBoundsException e) {System.out.println("Whoops!);}
       catch (NullPointerException e) {System.out.println("Double Whoops!");}}
    

• Question 4: Finding Errors -- indicate whether the code fails to compile, throws an exception on reasonable input, or has clearly unintended output (5 points each):

  • a)

    
       // new method in DogTeam
       public SledDog last ( ) {
          if (isEmpty( )) return null;
          SledDog temp = leadNode.getElement( );
          while (temp.getNext( ) != null) 
             temp = temp.getNext( );
          return temp;}
    

  • b)

    
    
       public class DogTeamDriver {
          DogTeam rcmp = new DogTeam( );
          for (int i = 0; i < 5; i++)
             rcmp.addToLead(new SledDog ("Dudley", 4, "Husky"));
          rcmp.addToFront(king);
          System.out.println(rcmp.countHuskies( ));}
    

  • c)

    
        // using L&C's code base
        QueueADT q = new LinkedQueue<Dog>( );
        q.enqueue(biscuit);
        q.enqueue(duncan);
        Dog temp = q.dequeue( );
        q.enqueue(cardie);
        while (!q.isEmpty( )) q.dequeue( );
        try {temp = q.peek( );}
        catch (EmptyCollectionException e) { }
        System.out.println(q.size( ));
    

• Question 5: Timing Analysis (in terms of n, we use L&C's code base) -- 5 points each
  • a)

    
        ArrayStack<Dog> s = new ArrayStack<Dog>( );
        for (int i = 0; i < n; i++)
           s.push(cardie);
    

  • b) Describe (in English or pseudocode) a method (to be added to the ArrayStack class) that returns the middle element of the calling ArrayStack. (This is the element that would be numbered n/2, where the size of the stack is n and "/" is division without remainder.) Analyze the worst-case running time of your method in terms of n. Make your method as efficient (in big-O terms) as possible.

  • c)

    
    
        // here "merge" is the method from 3b above
    
        DogTeam team1 = new DogTeam( );
        DogTeam team2 = new DogTeam( );
        for (int i = 0; i < n; i++) {
           team1.addToLead (buck);
           team2.addToLead (king);}
        for (int k = 0; k < n; k++) 
           team1.merge(team2);
    

• Question 6: Short Code Writing (10 points)

  A Maze is defined to be connected if every possible pair of open cells has at least one path from one to the other. Write an instance method public boolean isConnected( ), to be added to the Maze class, that returns true if and only if the calling Maze is connected. You should use existing methods from the Maze class, either from Project #2 or #4 (your choice).

• Question 7: Long Code Writing (20 points)

  The managers of the Iditarod sled dog race need to produce particular lists of dogs competing in the race, and would like to use your SledDog and DogTeam classes to do it. They have an array "entries" of DogTeam objects, containing all the dogs on all the teams entering the race. They want an array "ageTeams" containing the same dogs, but arranged into new teams where each team consists of dogs with the same age. You may assume that the ages of the dogs range from 0 through some constant MAX_AGE. The order of dogs within the new teams does not matter.

  Write a method public DogTeam[ ] makeAgeTeams (DogTeam [ ] entries) to go into one of their classes -- note that it thus may use only public methods of the class DogTeam. Assume that the DogTeams in "entries" are cloned before your method is run, so you need not worry about destroying the copies of them you have. (Hint: Put all the dogs into a queue, then take each dog in turn out of the queue and add it to the appropriate new team.)

Last modified 15 October 2011